Shadows on the Wall mainly consists of character sketches and proposed outlines that Art Wallace developed. Many of the details in those story lines were later changed on the show, so I don't think reading the book now would ruin the viewing for you. In fact, you may be pleasantly surprised when you finally do see the pre-Barnabas episodes to find how the episodes actually play out (e.g. what DS kept from the story bible, and what it changed.)
ProfStokes
